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Driven by the planned cessation of production of an existing platform, redevelopment of a subsea field currently tied back to the platform is underway. A floating host FPSO scheme that splits and re-uses the existing subsea system has been selected as the redevelopment solution.

The project includes the design, fabrication and installation of two Pipeline Bundles containing pipe-in-pipe production flowlines, gas lift flowlines and control systems. Additionally, it includes conditioning of existing pipelines, the fabrication of a 9km 16” gas export pipeline, flexible riser system, dynamic umbilical riser system and associated subsea tie-ins.

The subsea infrastructure is spread across two primary locations, the field feeding the FPSO and the gas export tie-in from the FPSO to an existing trunkline. In this operation, the task was to successfully commission the gas export line to permit future export of gas from the FPSO.

Historically methanol was used to remove moisture and so reduce the risk of gas hydrate formation but given issues with its flammability etc its use has declined over the years to be replaced with MEG. In this case the residual volumes of MEG remaining after commissioning the gas export pipeline could potentially cause problems at the receiving terminal and so a decision was made to revert back to methanol.

The challenge in this case was that the density would be decreasing as purity increased, in itself this would not have posed any real problem as sampling fluid as density rises or falls was already built into the design, but the pig train also had a volume of nitrogen between two of the pigs. Nitrogen density is obviously much less than any liquid and so a method was needed so that where the density fell to a very low level the system would not sample.

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